PS4/Pro will not be left on the market when PS5 releases (New games anyway), that is one way to cripple your new HW. Secondly, the difference between the two CPU's will be fucking huge.
With PS and MS both seeming to want to keep online games at the same frame rate for parity, if PS5 can do solid 60fps, it's hard to see them holding online multiplayer back to 30fps for third party games that cross gens. Unless PS4 players can't play with PS5 players, and I have a hard time believing that. It would make sense in terms of profits, because selling a $199 PS4 is going to keep hardware and software sales quite strong. Depends a lot on how much future games have to suffer on PS4's, or on PS5's to accommodate PS4.
Keeping PS4 Pro may not even be enough on 7nm to attain a solid 60fps, so focusing future games on PS5 makes sense. Except for the fact that your asking people to shell out another $399-$499 again. With MS seemingly talking about a much cheaper hardware based streaming box, to go along with a hardcore 4k/60 home console, I wouldn't put it passed PS to counter with a Pro/XB1X level PS5 base console using Ryzen to achieve 60fps for $299, while also having a (checkerboard) 4k/60 box for $399-$499.